This invention relates to cutting dies and more particularly to a unique variable position die set adapted for precisely cutting and trimming metal blanks subsequently wrapped to cylindrical shape.
In the manufacture of electric motor shells, it has been general industry practice to roll precut lengths of sheet metal into a cylindrical form. The ends of the rolled sheet metal are then welded. The internal diameter of the resulting cylindrical motor shell must be maintained within exact tolerances. This is necessary since it is common manufacturing practice to press fit the field laminations for the electric motors into place within the shell. If the motor shells are not formed with accurate circularity and with a constant, predetermined internal diameter, distortion of the field laminations may result. Such distortion presents variations in the air gap between the field and the rotor resulting in defective motor operation.
One apparatus and method for precisely forming cylindrical motor shells from flat blanks of sheet metal is disclosed in U.S. Pat. No. 3,732,614 to Boutell, entitled METHOD FOR MAKING MOTOR SHELLS AND THE LIKE, issued May 15, 1973, and assigned in common herewith. As shown therein, a precisely dimensioned and precision-ground forming arbor or mandrel is employed to produce the motor shells, by wrapping individual, precut lengths of sheet metal of an appropriate width around the forming arbor, using pivotally mounted jaw-like elements. Since the forming arbor has a diameter equal to the diameter of the field laminations for the electric motor, the resulting cylindrically shaped motor shell will possess the required internal diameter. Further, the shell will have accurate circularity, and therefore distortion of the field laminations of the motor is obviated. In the preferred present practice, the sheet metal blanks are precut in a cutoff and pierce die of the general type disclosed in the aforementioned U.S. patent. Although the sheet metal stock is typically supplied by its manufacturers with a specified nominal thickness, in stated ranges, the actual thickness of the individual cut blanks usually varies within the tolerances allowed by sheet metal manufacturers. In many industrial applications this variance in thickness is inconsequential and is generally ignored. However, in the manufacture of cylindrical sleeves or the like with controlled inside diameters, as in the case of motor shells, variances in the thickness of the sheet metal blanks result in changes in the internal diameter of the resulting shell if the shell is rolled so that the ends of the blank abut. This variation in the internal diameter of the shell causes serious quality control problems for their manufacturers, including a higher incidence of rejections and scrapped units as well as warranty claims for motors which fail prematurely.
If the shell manufacturer employs an apparatus of the type disclosed in the above-mentioned U.S. patent, as the thickness of the sheet stock varies the weld gap between the ends of the rolled or wrapped blank will likewise vary. For example, if the thickness of the blank is less than nominal stock thickness, the gap will decrease, and in fact it would not be unexpected for the ends of the wrapped blank to overlap, thus resulting in a defective shell. Further, if the thickness of the blank should be greater than the nominal stock thickness, the gap between the ends of the blank will increase. The increase may be sufficient to prevent effective welding of the ends of the wrapped blank. In either case, as the actual thickness of the sheet metal blanks varies from the nominal thickness, as is routinely the case, the resulting motor shell varies between two extremes, either of which is unacceptable.
A need therefore exists for an apparatus for precisely producing a sheet metal blank wherein the cut length may be varied as a function of the actual thickness of the sheet metal so that a cylinder of constant internal diameter having a constant weld gap may be produced.